The invention relates to the thermal gelation of foods and biomaterials, and more specifically, to the thermal gelation of foods and biomaterials using rapid heating. It is known in the art that some foods and biomaterials become hard as a result of boiling or frying, and the reason for this change is that the proteins coagulate and bind the components of the product together. It is also known that coagulation may be obtained by other types of heating such as microwave exposure.
There are several ways to expose food or biomaterial to microwave energy. For example, U.S. Pat. No. 4,237,145 to Risman et al. describes pumping eggs through a tube that is transparent to microwaves. U.S. Pat. No. 5,087,465 to Chen describes filling tubs with soybean milk and using a conveyor belt to carry the tubs through a microwave oven. U.S. Pat. No. 4,448,793 to Akesson describes filling a hollow mold with a meat paste and using two conveyor belts to pass the filled mold through a microwave waveguide.
One advantage of boiling or frying is that it is possible to use an equivalent point method to analyze the thermal effects on products. See U.S. Pat. No. 4,808,425 to Swartzel et al., which is hereby incorporated by reference. To determine the equivalent point of a thermal system, a complete thermal history of the treatment must be available. This is obtained by measuring mixed mean product temperatures at various locations (entrance to the heat exchanger, exit of the heat exchanger, and at least two locations inside the heat exchanger). Time is calculated by correlating mean residence time with location of the temperature probe. If it is difficult or impractical to insert thermal probes, time-temperature curves are calculated based on knowledge of the product's physical characteristics and on the geometry of the processing equipment.
There are three primary reasons that an equivalent point method has not been used with rapid heating, and more specifically microwaves. First, the microwave signal attenuates as it moves away from its source. As a result, the material is heated more at one end of the microwave than at the other end. This attenuation versus propagation distance increases as lossy materials are introduced. Second, because the magnitude of the electric field in the microwave signal has peaks and valleys due to forward and reverse propagation, the material is exposed to hot spots that heat the material unevenly. Third, there is a field gradient between conducting surfaces. As a result, materials near the conducting surface are heated less. A fourth reason is that some food products, i.e. food products high in fat, may require pretreatment at a lower temperature.
As explained in the '425 patent to Swartzel et al., treatment temperatures are primarily limited by the ability to accurately time the duration of the thermal treatment: as temperature is increased the treatment time must be decreased, and shorter treatment times are more difficult to administer with precision. As explained in more detail below, treatment times are also complicated by the length of the object to be heated. Utilizing the techniques discussed below, it is not only possible to use an equivalent point method in a microwave system, but it is also possible to achieve higher temperatures and shorter treatment times than previously thought possible. It is also possible to overcome the problems associated with longer objects. As a result, it is possible to achieve a safer product with a longer shelf live and the same or better texture (fracture stress and strain properties) in less time, less space, and with less product loss.